Which relationship shows a quadratic variation?
a. x 1 2 3 4
f(x) 4 3 2 1
b. x 1 2 3 4
f(x) 2 5 10 17
c. x 2 3 4 5
f(x) 1 4 9 16
d. x 1 2 3 4
f(x) 2 8 18 32